1. Field of the Invention
The present invention relates to RE-Ba-Cu-O type oxide superconductors exhibiting high critical current density at liquid nitrogen temperature, wherein RE means a combination of at least two elements selected from Nd, Sm, Eu, Gd, Y, Ho and Dy.
2. Description of the Related Art
Bulk process technology for preparing RE-Ba-Cu-O type crystals (RE is rare earth elements) has been considerably improved, which makes it possible to produce large RE-Ba-Cu-O oxide bulk superconductors having high critical current density and shifted crystal orientation.
Strong electromagnetic force can be generated by an interaction between such bulk superconductors and outer magnetic field under a contactless condition, which is widely applicable to a magnetic bearing and a contactless pump as well as a superconductive flywheel type electric power storage unit using low-loss magnetic bearings.
On the other hand, a large magnetic field can be trapped by the above mentioned superconductors due to considerable possibility of a pinning effect, and according to resent reports, there has been prepared bulk superconductors of about 1 T at liquid nitrogen temperature (77 K) and higher T values such as 10 T at a lower temperature of 45 K. As RE-Ba-Cu-O bulk superconductors trap a large magnetic field as described above, such superconductors in the same shape and size as a permanent magnet can generate much higher magnetic field than the magnet and attract attention as a novel functional material.
While RE-Ba-Cu-O type superconductors have improved critical current properties at relatively high temperature but difficulty in working characteristics, the bulk superconductors are tried to form into wires and tapes so as to widen application thereof.
It is the most desirable, however, to increase critical current density at higher temperature in a higher magnetic field in order to widen application of the bulk superconductors. For example, a magnetic field more than 5 T is necessary to use such superconductors as a magnet for magnetic levitation train which is now practically tested.
Further, an economic point of view is also important. From such a viewpoint, it is preferable to attain much higher magnetic field at higher temperature, if possible, at liquid nitrogen temperature.
As RE-Ba-Cu-O bulk superconductors tend to result in a typical pinning effect as described above, it is effective to homogeneously and finely disperse "normal conductive deposit", etc. which functions as a pinning center in the bulk superconductors to increase critical current density.
Conventional large sized RE-Ba-Cu-O bulk superconductors of expected high performance have been generally prepared by a "melting process". In this process, a RE-Ba-Cu-O superconductor composition as a starting composition is heated to form a semi-molten state, then seeded with a suitable seed crystal and slowly cooled to prepare a bulk material of shifted crystal orientation. It is also a general practice to provide temperature gradient to superconductors during a solidification step.
The "melting process" has been originally developed as technical method which is conducted in the air. Even now, when Y-, Ho- or Dy-containing superconductors (i.e., RE-Ba-Cu-O type superconductors wherein RE is Y, Ho or Dy) are prepared, a corresponding starting composition is generally subjected to melt-solidification in the air, while in the case of La-, Nd-, Sm-, Eu- or Gd-containing superconductors (i.e., RE-Ba-Cu-O superconductor wherein RE is La, Nd, Sm, Eu or Gd), melt-solidification of a starting composition is mostly carried out under a condition of lower oxygen partial pressure. The reason why is based on the following information. It has been found that "a cluster of slightly higher RE concentration and lower critical temperature compared with a parent phase" is spontaneously and finely dispersed in the parent phase to greatly raise critical current density, when RE-Ba-Cu-O superconductors having a solid solution composition of RE.sub.1+x Ba.sub.2-x Cu.sub.3 O.sub.y, wherein RE is La, Nd, Sm or Gd, are molten and grown under a condition of lower oxygen partial pressure.
In a resent "melting process" in which a RE-Ba-Cu-O bulk superconductor is prepared, wherein RE is La, Nd, Sm, Eu, Gd, Y, Ho or Dy, it has been found that, when a starting composition is melt-solidified, an initial composition thereof is set in advance to a RE.sub.2 BaCuO.sub.5 phase (so-called RE211 phase)-rich side, or to a RE.sub.4 Ba.sub.2 Cu.sub.2 O.sub.10 phase (so-called RE422 phase)-rich side in the case of RE.dbd.La or Nd, to successfully yield structure in which the RE211 or RE422 phase is finely dispersed in a superconductive REBa.sub.2 Cu.sub.3 O.sub.y. (RE123) phase. Using the above mentioned structure control technique, it has been also possible to prepare superconductors in which relatively higher critical current density is achieved at liquid nitrogen temperature.
When RE-Ba-Cu-O bulk superconductor are prepared, wherein RE is La, Nd, Sm Eu or Gd, it is expected to obtain a more preferable result because a cluster of higher RE concentration and the RE211 phase coexist in a parent phase of this system, both of which function as a pinning center so that the critical current density is increased.
In practice, however, the magnetic field dependence of critical current density at liquid nitrogen temperature is unexpectedly high because of larger particle size of the RE211 phase in the order of at least 2 to 10 .mu.m. As a result, an improvement in the critical current density can be achieved in the side of lower magnetic field because of the above mentioned function as a pinning center, while such an improvement is not so remarkable in the side of higher magnetic field and, in particular, when a magnetic field is impressed to a crystal parallel to the c axis thereof, the critical current density is considerably decreased in a magnetic field over 3T.
As described above, a contribution of the duster is essential for an improvement in the critical current density in the side of higher magnetic field. However, even if the duster contributes to the critical current density, such a contribution is not sufficient technically to provide "a large RE-Ba-Cu-O bulk superconductor" in which a high magnetic field enough to apply to a magnet for magnetic levitation train, etc. is obtained at relatively high temperature.